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Screening tests for genetic metabolic diseases remain extremely useful due to their rapidity, ease of interpretation and substantial reduction of cost. However, interferences in these tests are still a source of concern in laboratory medicine. Cost considerations have so compressed the duration of the medical work-up that the administration of radiologic contrast may often antedate the collection of body fluids for genetic-metabolic testing. It has been found that under these circumstances, certain contrast media may interfere with the urinary studies of amino acids, organic acids, and tests involving ultraviolet absorption such as those concerned with purines, pyrimidines, and related compounds. The consequences of interference may be misdiagnosis, repeated testing, extensive and expensive work-ups, and unnecessary delay and anxiety for the family. As in all testing, it is prudent to avoid medications and atypical diets, if possible. In the case of contrast media, it is a simple matter to collect samples for analysis prior to the administration of radiocontrast so as to avoid the pitfalls and yet not delay the diagnostic work-up.

An improved method for the determination of arylsulfatase A, based on the ether solubility of 4-nitrocatechol, has been established. The method has been applied to the diagnosis of metachromatic leukodystrophy and can identify the carrier state. The method is also useful in organ surveys for the enzyme.